They tend to do the "no true scotsman" fallacy a lot. By that, i mean if you bring up any examples that didn't work out they'll say "that's not true communism, nobody has done true communism before but it'll be sweet and perfect and there won't be any issues, and if you're skeptical you're a monster. Just take it on faith that this entirely untested thought-experiment system is going to go awesomely", despite them supposedly being materialists and evidence-based and opposing faith-based religions.
The real problem with pure communism is that it relies on everyone always acting on good faith. Game theory explains why it won't work that way. The Prisoner's Dilemma + the Nash Equilibrium basically disprove that both right-wing libetarianism and completely non-authoritarian communism can ever work. Even if everyone's better off working together, that is unstable, since using game theory + the Nash Equilibrium you can show that an individual always improves their own position by cheating (working outside the public ownership thing in communism for example, or ignoring regulations in the free market system).
The Nash Equilibrium stuff basically shows the flaw in the reasoning that "since everyone will be better off working together in communism then once we have communism, then people will realize they're better off and continue to work together". Yeah, even if the "better off working together" thing is true, the "people will continue working together" part isn't true. If it was true, people would already be working together. It's not like "cooperating" is a novel concept unique to communism. No, the problem is that cooperating is mathematically unstable, as John Nash had shown, even if it's the globally optimal solution. As long as people have a choice.
The reason that it won't work is that cooperating is only stable if you force people to cooperate and not do anything in their immediate self-interest. This prevents people from being allowed to make the choices that lead to the Nash Equilibrium. But removing choice is not the optimal way to ensure a good distribution of resources, and the reason for this is that it's failed to account for the value of having choice - so it's failed to account for an externality.
The problem with *everything being owned by the public is that you constantly have to police that at a very detailed level to ensure people don't make stuff by themselves, since that's a way around the restrictions. So sure, say that in this grand communist future we get it so we only have to work for the system for 8 hours a week, and that provides us enough of everything we need to live on. What are people going to do with the rest of their week then? Pure leisure? If they create *anything* of value in the rest of that time that's outside of the control of the collective, that becomes a threat to the basic tenets of the collective, that it controls the means of production. So it ends up inevitably at a form of police state where you're heavily regulated and monitored in what you can and cannot do. You'll be regulated *a bit* at work, but you'll be regulated even more when not at work, because it's what you're doing while not officially working that is the biggest threat to the system. That's why this is not a stable outcome.